Oligonucleotides are often modified or labeled with reporter moieties such as quenchers, fluorophores, biotin, etc. These labeled oligonucleotides can provide information regarding binding and other biological phenomena, the structure of DNA, the association of macromolecules, and the size and mobility of protein and DNA complexes.
Several attachment chemistries are currently used for modifying oligonucleotides. For example, primary amino groups are widely used to attach modifiers, reporter moieties or labels to an oligonucleotide. In addition, they can be used to attach an oligonucleotide to a solid surface.
Stable Schiff base linkers have been used for the synthesis of labeled oligonucleotides. (Dey & Sheppard (2001) Org. Lett. Vol. 3, 25:3983-3986, which is incorporated herein by reference). The methods have been limited to the post-synthetic attachment of labels, and the proposed methods have not been commercially viable alternatives to standard synthesis approaches. Previously described post-synthetic methods permit the incorporation of only a single type of reporter moiety or multiple copies of the same reporter moiety into an oligonucleotide.
Labeled oligonucleotides have a wide variety of useful applications. For example, light quenching processes that rely on the interaction of a fluorophore and quencher as their spatial relationship changes can be used in convenient processes for detecting and/or identifying oligonucleotides and other biological phenomena. In one such method, the change in fluorescence of a fluorophore or quencher can be monitored as two oligonucleotides (one containing a fluorophore and one containing a quencher) hybridize to each other. The hybridization can be detected without intervening purification steps that separate unhybridized from hybridized oligonucleotides. Currently, quencher groups are commonly placed at the end of a probe sequence while the fluorophore is placed at the opposite end, solely for ease of synthesis. However, in some applications, such as real-time PCR, dual-labeled probes are more effective when the labels are placed closer to each other.
Perhaps the most common mechanism of fluorescent quenching is fluorescent resonance energy transfer (“FRET”). For FRET to occur, a fluorophore and a fluorescent quencher must be within a suitable distance for the quencher to absorb energy from the donor. In addition, there must be overlap between the emission spectrum of the fluorescent donor and the absorbance spectrum of the quencher. This requirement complicates the design of probes that utilize FRET because not all potential quencher/fluorophore pairs can be used. For example, the quencher known as BHQ-1, which absorbs light in the wavelength range of about 520-550 nm, can quench the fluorescent light emitted from the fluorophore, fluorescein, which fluoresces maximally at about 520 nm. In contrast, the quencher BHQ-3, which absorbs light in the wavelength range of about 650-700 nm would be almost completely ineffective at quenching the fluorescence of fluorescein through FRET but would be quite effective at quenching the fluorescence of the fluorophore known as Cy5 which fluoresces at about 670 nm.
Oligonucleotides labeled with fluorophores and quenchers can also be used to monitor the kinetics of PCR amplification. For example, a PCR reaction is performed using oligonucleotides designed to hybridize to the 3′ side (“downstream”) of an amplification primer so that the 5′-3′ exonuclease activity of a polymerase digests the 5′ end of the probe, cleaving off one of the dyes. The fluorescence intensity of the sample increases and can be monitored as the probe is digested during the course of amplification.
Similar oligonucleotide compositions may be used in other molecular/cellular biology and diagnostic assays, such as end-point PCR, in situ hybridizations, in vivo DNA and RNA species detection, single nucleotide polymorphism (SNPs) analysis, enzyme assays, and in vivo and in vitro whole cell assays.